Query transformation

ABSTRACT

A mechanism is provided for transforming an original database query into a supported database query that can be fully computed by a target database. The original database query comprising a select list including a plurality of expressions, the plurality of expressions having a control break. The plurality of expressions includes an expression that cannot be directly computed by the target database. The mechanism constructs a derived table from the unsupported database query comprising constructing a new select list of the derived table, traversing the plurality of expressions of the select list of the unsupported database query, and adding a GROUP BY expression to the derived table based on the new select list of the derived table. The mechanism constructs the supported database query using the unsupported database query and the derived table.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application claiming priorityfrom U.S. application Ser. No. 10/856,306, filed May 27, 2004, now U.S.Pat. No. 7,428,532, the content of which is incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The invention relates generally to data access middleware, and inparticular to a system and method of query transformation.

BACKGROUND OF THE INVENTION

A typical data access environment has a multi-tier architecture. Fordescription purposes, it can be separated into three distinct tiers:

-   -   Web server    -   Applications    -   Data        The tiers are based on business function, and are typically        separated by firewalls. Client software, such as a browser or a        report-authoring tool, sits above the tiers.

The web server contains a firewall and one or more gateways. All webcommunication is performed through a gateway. A gateway is responsiblefor passing on requests to the application server, in tier 2, forexecution.

The applications tier contains one or more application servers. Theapplication server runs requests, such as reports and queries that areforwarded by a gateway running on the web server. Typically, one of thecomponents of the applications tier is a query engine, which is dataaccess middleware that provides universal data access to a variety ofheterogeneous database systems. The query engine formulates queries(typically SQL) and passes them on to the data tier, through a nativedatabase API (such as ODBC) for execution.

The data tier contains database management systems (DBMS), which manageraw data stored in a database. Examples of such systems include Oracle,DB2, and Microsoft SQL Server.

Although a multi-tier architecture can be configured in severaldifferent ways, a typical configuration places each tier on a separatecomputer (server). A database server is typically a “high end” server,and thus can process queries at a relatively fast speed. An applicationserver cannot generally process queries as quickly as a database server.

In order to solve many business questions, a query engine may generateSQL queries that utilize the SQL/OLAP technology introduced in the SQL:1999 standard. However, many database systems do not support thistechnology. Thus, the SQL queries would have to be performed by thequery engine on the application server that is generally slower than thedatabase server. It is desirable to have as much processing performed onthe database server as possible.

There is a need to prevent or reduce the amount of local (applicationserver) processing required to process a query. In the past, theapplication would be responsible for generating SQL that contained astandard GROUP BY query. Quite often, generating this type of SQL ismore difficult since it is more complex.

One way of overcoming this problem is for the query engine to generate aGROUP BY query to compute aggregates at the lowest level of granularity,and then compute the remaining aggregates based on these values. Thisapproach reduces the amount of processing time required on theapplication server.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofClient/Server Aggregate query transformation in a database system thatdoes not support SQL: 1999 standard.

In accordance with an embodiment of the present invention, there isprovided a system for transforming client/server aggregates. The systemcomprises a client/server analysis component for analyzing one or morequeries that are not supported by a target database system, and aclient/server transformation component for transforming the one or morequeries into semantically equivalent queries that are supported by thetarget database system.

In accordance with another embodiment of the present invention, there isprovided a method of client/server aggregate transformation. The methodcomprises the steps of analyzing one or more queries that are notsupported by a target database system, and transforming the one or morequeries into semantically equivalent queries that are supported by thetarget database system.

In accordance with another embodiment of the present invention, there isprovided a method of client/server aggregate transformation. The methodcomprises the steps of analyzing a client/server select list todetermine if a transformation is to be performed, generating a derivedtable, traversing all expressions in the main select list, addingcolumns or expressions that are grouping columns to the derived tableselect list, and converting and moving eligible aggregates into thederived table select list.

In accordance with another embodiment of the present invention, there isprovided a computer data signal embodied in a carrier wave andrepresenting sequences of instructions which, when executed by aprocessor, cause the processor to perform a method of client/serveraggregate transformation. The method comprises the steps of analyzingone or more queries that are not supported by a target database system,and transforming the one or more queries into semantically equivalentqueries that are supported by the target database system.

In accordance with another embodiment of the present invention, there isprovided a computer-readable medium having computer readable codeembodied therein for use in the execution in a computer of a method ofclient/server aggregate transformation. The method comprises the stepsof analyzing one or more queries that are not supported by a targetdatabase system, and transforming the one or more queries intosemantically equivalent queries that are supported by the targetdatabase system.

In accordance with another embodiment of the present invention, there isprovided a computer program product for use in the execution in acomputer of a query transformation system for transforming client/serveraggregates. The computer program product comprises a client/serveranalysis component for analyzing one or more queries that are notsupported by a target database system, and a client/servertransformation component for transforming the one or more queries intosemantically equivalent queries that are supported by the targetdatabase system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical data access environment.

FIG. 2 shows a client/server aggregate transformation system, inaccordance with an embodiment of the present invention.

FIG. 3 shows in a flowchart an example of a method of client/serveraggregate transformation, in accordance with the client/server aggregatetransformation system.

FIG. 4 shows in a flowchart another example of a method of client/serveraggregate transformation, in accordance with the client/server aggregatetransformation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical data access environment 10 for processing data.Typically, data is stored in a database 11. A DBMS running on a databaseserver 12 accesses the raw data stored in the database 11. A queryengine 15, running on a report server (or application server) 13 is usedto generate reports on the raw data and instruct the DBMS on thedatabase server 12 to obtain information pertaining to the raw data inthe database 11. The query engine 15 provides universal data access to avariety of heterogeneous database systems. An end user uses a clientapplication 14, running on a client workstation, to facilitateapplication server 13 operations.

In order to solve many business questions, a query engine 15 generatesSQL queries that utilize the SQL/online analytical programming (OLAP)technology introduced in the SQL: 1999 standard. These SQL queriesinclude SQL/OLAP functions (windowed aggregates). However, many databasesystems 12 do not support this technology. In order to prevent or reducethe amount of local (application server) processing required to processthese types of queries, the query engine 15 attempts to generatesemantically equivalent queries that can be processed in whole or inpart on the database server 12 by the target database system. Thesesemantically equivalent queries involve standard aggregate functions andthe GROUP BY operator.

FIG. 2 shows, a Client/Server Aggregate transformation system 20, inaccordance with an embodiment of the present invention. TheClient/Server Aggregate transformation system 20 comprises aClient/Server analysis module 21 for analysing SQL/OLAP queries that arenot supported by a target database system, and a Client/Servertransformation module 22 for transforming these SQL/OLAP queries intosemantically equivalent queries that are supported by the targetdatabase system.

The Client/Server Aggregate transformation system 20 is implemented as asub-system of the query engine 15 in the data access environment 10.This transformation 20 may generate queries that can be processed intheir entirety on the database server 12, or queries that requireprocessing on both the application server 13 and the database server 12.

Advantageously, the Client/Server Aggregate transformation system 20reduces processing that might otherwise be required on the applicationserver by generating a semantically equivalent query, thereby improvingperformance in many cases. Furthermore, the Client/Server Aggregatetransformation system 20 takes advantage of functionality provided bythe target database. In particular, the Client/Server Aggregate system20 utilizes the functionality provided by standard aggregates and theGROUP BY operator.

There are two types of OLAP functions: framed functions and reportfunctions. Framed OLAP functions contain a window frame specification(ROWS or RANGE) and an ORDER BY clause. Through window frames,capabilities such as cumulative (running) sums and moving averages canbe supported. Report functions do not contain a window framespecification, and produce the same value for each row in a partition.

The Client/Server Aggregate transformation attempts to reduce the numberof derived tables generated in a SQL group transformation by computingsome of the aggregates locally. This transformation can be utilized whenthe user is willing to tolerate some (local) processing on theapplication server.

FIG. 3 shows a flowchart of an example of a method of Client/ServerAggregate transformation (30), in accordance with an embodiment of theClient/Server Aggregate transformation system 20. The method (30) beginswith analysing a query containing a client/server aggregate that is notsupported by a target database system (31). Next, the query istransformed into a semantically equivalent query that is supported bythe target database system (32). The method (30) is done (33).

FIG. 4 shows a flowchart of an example of a method of Client/ServerAggregate transformation (40), in accordance with an embodiment of theClient/Server Aggregate transformation system 20. The method (40) beginswith analyzing a select list to determine if transformations can beperformed (41). If transformations cannot be performed (42), the method(40) is done (46). If a transformation is to be performed (42), aderived table is generated (43). All (grouping) columns of the longestcontrol break are added to the select list of the derived table (44). Acontrol break is a set of columns over which a particular OLAP functionis computed. A partition is a set of rows that have the same value forcolumns that make up the control break. Then the select list istraversed and candidate aggregates are convened and moved into theselect list of the derived table (45). The method (40) is done (46).

To determine if the transformation can be performed, all expressions andaggregates in the select list are analyzed. This analysis consists ofthe following:

-   -   Assigning a group index. This index indicates whether the        aggregate is eligible for transformation. Any aggregate assigned        a group index of −1 is not eligible. Report aggregates are        eligible, provided they have a standard aggregate counterpart        (MIN, MAX, SUM, AVG, COUNT, COUNT(*)). Framed aggregates are not        eligible.    -   Keeping track of all control breaks. Control break information        is stored in an ordered list, with control breaks having the        fewest number of columns (least amount of detail) appearing        first.        The transformation can be performed if the following criteria        are met:    -   The keyword DISTINCT is present in the select list.    -   All aggregates in the select list are eligible.    -   All control breaks are compatible. Assuming that the longest        control break G is of the form (C₁, C₂, . . . , C_(n)), all        other control breaks must be of the form (C₁, C₂, . . . ,        C_(k)), where 0≦k≦n. Note that the control break 0 is compatible        with all control breaks.    -   Any simple column reference in the select list that is not part        of an aggregate specification is contained in the G, or is part        of an expression contained in G.

In performing the transformation, the following assumptions are made:

-   -   m equals the total number of unique control breaks    -   n equals the number of columns in the longest control break    -   G represents the list of columns in the longest control break,        known as the set of grouping columns    -   AGG (C₀) represents the standard form of the aggregate AGG(C₀)        OVER( . . . )

Given these assumptions, the basic steps in performing thetransformation are as follows:

-   -   1. Construct a new select list (known as die inner select list)        to hold columns and standard aggregates.    -   2. Traverse all expressions in the original select list.    -   3. If the expression is a simple column reference or it is        contained in G, copy it to the inner select list. Otherwise,        traverse the expression, and apply the rules outlined here.    -   4. Apply one of the following transformations if m>1 or k≠n.        -   a. If the expression is an aggregate is of the form AVG(C₀)            OVER( ), replace it with an expression of the form            SUM(SUM(C₀)) OVER( )/SUM(COUNT(C₀)) OVER( ) and move the            standard (nested) aggregates into the inner select list.        -   b. If the expression is an aggregate is of the form AVG(C₀)            OVER(PARTITION BY C₁, C₂, . . . , C_(k)), replace it with an            expression of the form SUM(SUM(C₀) OVER(PARTITION BY C₁, C₂,            . . . , C_(k))) OVER( )/SUM(COUNT(C₀) OVER(PARTITION BY C₁,            C₂, . . . , C_(k))) OVER( ) and move the standard (nested)            aggregates into the inner select list.    -   5. If the expression is an aggregate of the form AGG(C₀)        OVER(PARTITION BY C₁, C₂, . . . , C_(n)), replace it with        AGG(C₀), and move it into the inner select.    -   6. If the expression is an aggregate of the form AGG(C₀)        OVER(PARTITION BY C₁, C₂, . . . , C_(k)), where k≠n, replace it        with AGG(AGG(C₀)) OVER(PARTITION BY C₁, C₂, . . . , C_(k)), and        move the standard (nested) aggregate into the inner select list.    -   7. Construct the final query. This consists of constructing a        derived table containing a GROUP BY query for computing the        standard aggregates in the inner select list.

For the purpose of discussion, assume the target database for thefollowing examples does not support the SQL/OLAP technology introducedin the SQL: 1999 standard. The native SQL shown in each example is theSQL passed to the target database.

EXAMPLE 1

In this example, multiple SUM functions with compatible control breaksare present, so the transformation is straightforward.

Original Query SELECT DISTINCT SNO, PNO, SUM( QTY ) OVER ( ), SUM( QTY )OVER ( PARTITION BY SNO ), SUM( QTY ) OVER ( PARTITION BY SNO, PNO )FROM SUPPLY

Transformed Query SELECT C0, C1, SUM( C2 ) OVER ( ), SUM( C2 ) OVER (PARTITION BY SNO ), C2 FROM ( SELECT SNO C0, PNO C1, SUM( QTY ) C2 FROMSUPPLY GROUP BY SNO, PNO ) T1

Native SQL SELECT SNO C0, PNO C1, SUM( QTY ) C2 FROM SUPPLY GROUP BYSNO, PNOExplanation

The original query contains OLAP SUM functions computed over thepartitions ( ), (SNO), and (SNO, PNO). Because of the presence of theDISTINCT keyword, and the fact that the detail columns (SNO and PNO) arepan of a PARTITION BY clause, the query can be rewritten using a singlederived table that computes the sum with the finest granularity (SNO,PNO). The remaining SUM functions can be computed based on this value,as shown in the transformed query.

EXAMPLE 2

In this example, multiple SUM functions with compatible control breaksare present, but an AVG function has been introduced, making thetransformation a little more complex.

Original Query SELECT DISTINCT SNO, PNO, SUM( QTY ) OVER ( ), SUM( QTY )OVER ( PARTITION BY SNO ), SUM( QTY ) OVER ( PARTITION BY SNO, PNO ),AVG( QTY ) OVER ( ) FROM SUPPLY

Transformed Query SELECT C0, C1, SUM( C2 ) OVER ( ), SUM( C2 ) OVER (PARTITION BY SNO ), C2, SUM( C2 ) OVER ( ) / SUM( C3 ) OVER ( ) FROM (SELECT SNO C0, PNO C1, SUM( QTY ) C2, COUNT( QTY ) C3 FROM SUPPLY GROUPBY SNO, PNO ) T1

Native SQL SELECT SNO C0, PNO C1, SUM( QTY ) C2, COUNT( QTY ) C3 FROMSUPPLY GROUP BY SNO, PNOExplanation

The original query contains OLAP SUM functions computed over thepartitions ( ), (SNO), and (SNO, PNO). It also contains an OLAP AVGfunction computed over the partition ( ). Because of the presence of theDISTINCT keyword, and the fact that the detail columns (SNO and PNO) arepan of a PARTITION BY clause, the query can be rewritten using a singlederived table that computes the sum with the finest granularity (SNO,PNO). The remaining SUM functions can be computed based on this value,as shown in the transformed query. This value can be used to compute theaverage as well. However, we also require a count to be computed at thelowest level of granularity. The final average is then computed as shownin the transformed query (average of an average will not work).

EXAMPLE 3

In this example, the OLAP functions SUM and AVG have identical controlbreaks corresponding to the required detail information (SNO, PNO), sothe entire query can be replaced with a simple GROUP BY query.

Original Query SELECT DISTINCT SNO, PNO, SUM( QTY ) OVER ( PARTITION BYSNO, PNO ), AVG( QTY ) OVER ( PARTITION BY SNO, PNO ), FROM SUPPLY

Transformed Query SELECT SNO C0, PNO C1, SUM( QTY ) C2, AVG( QTY ) C3FROM SUPPLY GROUP BY SNO, PNO

Native SQL SELECT SNO C0, PNO C1, SUM( QTY ) C2, AVG( QTY ) C3 FROMSUPPLY GROUP BY SNO, PNOExplanation

The original query contains two OLAP functions computed over thepartition (SNO, PNO). This partition represents the grouping columns inthe transformed query. Because of the presence of the DISTINCT keyword,and the fact that the detail columns (SNO and PNO) are grouping columns,the query can be rewritten as a simple GROUP BY query.

EXAMPLE 4

In this example, the OLAP functions SUM and AVG have identical controlbreaks corresponding to the required detail information (SNO, PNO), sothe entire query can be replaced with a simple GROUP BY query.

Original Query SELECT DISTINCT SNO, PNO, SUM( QTY ) OVER ( ), SUM( QTY )OVER ( PARTITION BY SNO ) FROM SUPPLY

Transformed Query SELECT C0, C1, SUM( C2 ) OVER ( ), SUM( C2 ) OVER (PARTITION BY C0 ) FROM ( SELECT SNO C0, PNO C1, SUM( QTY ) C2 FROMSUPPLY GROUP BY SNO, PNO ) T1

Native SQL SELECT SNO C0, PNO C1, SUM( QTY ) C2 FROM SUPPLY GROUP BYSNO, PNOExplanation

The original query contains two SUM functions computed over thepartitions ( ) and (SNO); The longest control break (SNO), however, doesnor correspond to the detail information required (SNO, PNO). Hence, thederived table generated consists of a GROUP BY query that computes a sumgrouped on (SNO, PNO). Both of the outer sum values are then computedbased on this value.

A system for query transformation may comprise an analysis component anda transformation component. The analysis component may comprise a NestedAggregate analysis module, a SQL GROUP transformation analysis module, asummary filter transformation analysis module, and a Summary Filleranalysis module. The transformation component may comprise a NestedAggregate transformation module, a SQL GROUP transformation module, aClient/Server Aggregate transformation module, and a Summary Filtertransformation module.

The systems and methods according to the present invention may beimplemented by any hardware, software or a combination of hardware andsoftware having the functions described above. The software code, eitherin its entirety or a pan thereof, may be stored in a computer readablememory. Further, a computer data signal representing the software codethat may be embedded in a carrier wave may be transmitted via acommunication network. Such a computer readable-memory and a computerdata signal are also within the scope of the present invention, as wellas the hardware, software and the combination thereof.

While particular embodiments of the present invention have been shownand described, changes and modifications may be made to such embodimentswithout departing from the true scope of the invention.

1. A method of transforming an unsupported database query into a supported database query that can be fully computed by a target database, the unsupported database query comprising a select list including a plurality of expressions, the plurality of expressions each having a control break, the plurality of expressions including an expression that cannot be directly computed by the target database, the method comprising: constructing a derived table from the unsupported database query, wherein constructing the derived table comprises: constructing a new select list of the derived table; traversing the plurality of expressions of the select list of the unsupported database query and for each of the expressions: determining a corresponding expression to add to the new select list, wherein determining the corresponding expression to add to the new select list comprises determining an aggregate of the expression and determining a control break of the aggregate; adding the corresponding expression to the new select list; adding the control break of the expression to an ordered list of control breaks, wherein the ordered list is ordered from a shortest control break with a fewest number of columns to a longest control break with a highest number of columns; and when the control break of the aggregate has fewer columns than the longest control break of the order list of control breaks, and the aggregate of the expression is of the form AGG(C₀): replacing the expression AGG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(k)) with an expression of the form AGG(AGG(C₀)) OVER(PARTITION C₁, C₂, . . . , C_(k)), where each C is a column, where n is a number of columns in the longest control break, and where k is a number of columns in the other control break; and adding a GROUP BY expression to the derived table based on the new select list of the derived table; determining the unsupported database query can be transformed into a supported database query, wherein determining the unsupported database query can be transformed into the supported database query comprises determining all control breaks of the unsupported database query are compatible based on the ordered list; and constructing the supported database query using the unsupported database query and the derived table.
 2. The method as claimed in claim 1, wherein constructing the supported database query comprises: adding to a select list of the supported database query, expressions of the select list of the unsupported database query with a standard expression referencing the corresponding expression added to the new select list of the derived table; and replacing a FROM table of the unsupported database query with a reference to the derived table.
 3. The method as claimed in claim 1, further comprising: for each expression, determining the control break of the expression is not already in the ordered list prior to adding the control break of the expression to the ordered list.
 4. The method as claimed in claim 1, the method further comprising, when the control break of the aggregate of the expression has no columns, and the aggregate of the expression is of the form AVG(C₀): replacing the expression AVG(C₀) OVER ( ) with an expression of the form SUM(SUM(C₀)) OVER( )/SUM(COUNT(C₀)) OVER( ), where C₀ is a column.
 5. The method as claimed in claim 4, further comprising, when the control break of the aggregate of the expression has one or more columns found in the longest control break in the ordered list, and the aggregate of the expression is of the form AVG(C0): replacing the expression AVG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) with an expression of the form SUM(SUM(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) OVER( )/SUM(COUNT(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) OVER( ), where each C is a column and where n is the number of columns in the longest control break.
 6. The method as claimed in claim 1, the method further comprising, when the control break of the aggregate is the same as the longest control break of the order list of control breaks, and the aggregate of the expression is of the form AGG(C₀): replacing the expression AGG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n)) with an expression of the form AGG(C₀), where each C is a column and where n is the number of columns in the longest control break.
 7. The method as claimed in claim 1, where aggregates of the form AGG comprise one of MIN, MAX, SUB, COUNT.
 8. The method as claimed in claim 1, wherein determining the unsupported database query can be transformed into a supported database query further comprises: determining a DISTINCT keyword is present in the select list of the unsupported database query; determining all aggregates of the expressions are eligible for transformation; and determining simple column references in the select list of the unsupported database query are found in the longest control break of the unsupported database query, when the simple column references are not part of a speciation of one of the aggregates of the unsupported database query.
 9. The method as claimed in claim 8, wherein determining an aggregate is eligible comprises: determining the aggregate is a report aggregate that has a standard aggregate counterpart.
 10. The method as claimed in claim 8, wherein determining all control breaks are compatible comprises: determining the longest control break is of the form (C₁, C₂, . . . , C_(n)) and all other control breaks are of the form ( ) or of the form (C₁, C₂, . . . , C_(k)) where 0<=k<=n, where: each C is a column; n is the number of columns in the longest control break; and k is the number of columns in the other control break.
 11. A computer program product comprising a computer readable memory having computer readable code embodied therein for use in the execution in a computer of a method of transforming an unsupported database query into a supported database query that can be fully computed by a target database, the unsupported database query comprising a select list including a plurality of expressions, the plurality of expressions each having a control break, the plurality of expressions including an expression that cannot be directly computed by the target database, the computer comprising a processor coupled to the computer storage medium for executing the computer readable code, the method comprising: constructing a derived table from the unsupported database query, wherein constructing a derived table from the unsupported database query comprises: constructing a new select list of the derived table; traversing the plurality of expressions of the select list of the unsupported database query and for each of the expressions: determining a corresponding expression to add to the new select list, wherein determining the corresponding expression to add to the new select list comprises determining an aggregate of the expression and determining the control break of the aggregate; adding the corresponding expression to the new select list; adding the control break of the expression to an ordered list of control breaks, wherein the ordered list is ordered from a shortest control break with a fewest number of columns to a longest control break with a highest number of columns; and when the control break of the aggregate has fewer columns than the longest control break of the order list of control breaks, and the aggregate of the expression is of the form AGG(C₀): replacing the expression AGG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(k)) with an expression of the form AGG(AGG(C₀)) OVER (PARTITION C₁, C₂, . . . , C_(k)), where each C is a column, where n is a number of columns in the longest control break, and where k is a number of columns in the other control break; and adding a GROUP BY expression to the derived table based on the new select list of the derived table; determining the unsupported database query can be transformed into a supported database query, wherein determining the unsupported database query can be transformed into a supported database query comprises determining all control breaks of the unsupported database query are compatible based on the ordered list; and constructing the supported database query using the unsupported database query and the derived table.
 12. The computer program product as claimed in claim 11, wherein constructing the supported database query comprises: adding to a select list of the supported database query, expressions of the select list of the unsupported database query with a standard expression referencing the corresponding expression added to the new select list of the derived table; and replacing a FROM table of the unsupported database query with a reference to the derived table.
 13. The computer program product as claimed in claim 11, the method further comprising: when the control break of the aggregate of the expression has no columns, and the aggregate of the expression is of the form AVG(C₀), replacing the expression AVG(C₀) OVER ( ) with an expression of the form SUM(SUM(C₀)) OVER( )/SUM(COUNT(C₀)) OVER( ) where C₀ is a column; and when the control break of the aggregate of the expression has one or more columns found in the longest control break in the ordered list, and the aggregate of the expression is of the form AVG(C0), replacing the expression AVG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) with an expression of the form SUM(SUM(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) OVER( )SUM(COUNT(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n)))OVER( ), where each C is a column and where n is the number of columns in the longest control break.
 14. The computer program product as claimed in claim 11, the method further comprising: when the control break of the aggregate is the same as a longest control break of the order list of control breaks, and the aggregate of the expression is of the form AGG(C₀), replacing the expression AGG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n)) with an expression of the form AGG(C₀), where each C is a column and where n is the number of columns in the longest control break.
 15. The computer program product as claimed in claim 11, wherein determining the unsupported database query can be transformed into a supported database query further comprises: determining a DISTINCT keyword is present in the select list of the unsupported database query; determining all aggregates of the expressions are eligible for transformation; and determining simple column references in the select list of the unsupported database query are found in the longest control break of the unsupported database query, when the simple column references are not part of a speciation of one of the aggregates of the unsupported database query, wherein determining an aggregate is eligible comprises determining the aggregate is a report aggregate that has a standard aggregate counterpart; and wherein determining all control breaks are compatible comprises: determining the longest control break is of the form (C₁, C₂, . . . , C_(n)) and all other control breaks are of the form ( ) or of the form (C₁, C₂, . . . , C_(k)) where 0<=k<=n, where: each C is a column; n is the number of columns in the longest control break; and k is the number of columns in the other control break.
 16. A computer comprising: a processor; and a computer readable memory having computer readable code embodied therein for use in the execution by the processor of a method of transforming an unsupported database query into a supported database query that can be fully computed by a target database, the unsupported database query comprising a select list including a plurality of expressions, the plurality of expressions each having a control break, the plurality of expressions including an expression that cannot be directly computed by the target database, the method comprising: constructing a derived table from the unsupported database query, wherein constructing a derived table from the unsupported database query comprises: constructing a new select list of the derived table; traversing the plurality of expressions of the select list of the unsupported database query and for each of the expressions: determining a corresponding expression to add to the new select list, wherein determining the corresponding expression to add to the new select list comprises determining an aggregate of the expression and determining the control break of the aggregate; adding the corresponding expression to the new select list; adding the control break of the expression to an ordered list of control breaks, wherein the ordered list is ordered from a shortest control break with a fewest number of columns to a longest control break with a highest number of columns; when the control break of the aggregate has fewer columns than the longest control break of the order list of control breaks, and the aggregate of the expression is of the form AGG(C₀): replacing the expression AGG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(k)) with an expression of the form AGG(AGG(C₀)) OVER (PARTITION C₁, C₂, . . . , C_(k)), where each C is a column, where n is a number of columns in the longest control break, and where k is a number of columns in the other control break; and adding a GROUP BY expression to the derived table based on the new select list of the derived table; determining the unsupported database query can be transformed into a supported database query, wherein determining the unsupported database query can be transformed into a supported database query comprises determining all control breaks of the unsupported database query are compatible based on the ordered list; and constructing the supported database query using the unsupported database query and the derived table.
 17. The computer as claimed in claim 16, the method further comprising: when the control break of the aggregate of the expression has no columns, and the aggregate of the expression is of the form AVG(C₀), replacing the expression AVG(C₀) OVER ( ) with an expression of the form SUM(SUM(C₀)) OVER( )/SUM(COUNT(C₀)) OVER( ) where C₀ is a column; and when the control break of the aggregate of the expression has one or more columns found in the longest control break in the ordered list, and the aggregate of the expression is of the form AVG(C0), replacing the expression AVG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) with an expression of the form SUM(SUM(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n))) OVER( )SUM(COUNT(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n)))OVER( ) where each C is a column and where n is the number of columns in the longest control break.
 18. The computer as claimed in claim 16, the method further comprising: when the control break of the aggregate is the same as the longest control break of the order list of control breaks, and the aggregate of the expression is of the form AGG(C₀), replacing the expression AGG(C₀) OVER (PARTITION BY C₁, C₂, . . . , C_(n)) with an expression of the form AGG(C₀), where each C is a column and where n is the number of columns in the longest control break.
 19. The computer as claimed in claim 16, wherein determining that the unsupported database query can be transformed into a supported database query further comprises: determining a DISTINCT keyword is present in the select list of the unsupported database query; determining all aggregates of the expressions are eligible for transformation; and determining simple column references in the select list of the unsupported database query are found in the longest control break of the unsupported database query, when the simple column references are not part of a speciation of one of the aggregates of the unsupported database query, wherein determining an aggregate is eligible comprises determining that the aggregate is a report aggregate that has a standard aggregate counterpart; and wherein determining all control breaks are compatible comprises: determining the longest control break is of the form (C₁, C₂, . . . , C_(n)) and all other control breaks are of the form ( ) or of the form (C₁, C₂, . . . , C_(k)) where 0<=k<=n, where: each C is a column; n is the number of columns in the longest control break; and k is the number of columns in the other control break. 